King of JavaScript Online HTML Editor

<!DOCTYPE html>
<html lang="en">
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Calculator App with JavaScript</title>
<style>
html, body, div, span, applet, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
a, abbr, acronym, address, big, cite, code,
del, dfn, em, img, ins, kbd, q, s, samp,
small, strike, strong, sub, sup, tt, var,
b, u, i, center,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, embed, 
figure, figcaption, footer, header, hgroup, 
menu, nav, output, ruby, section, summary,
time, mark, audio, video {
    margin: 0;
    padding: 0;
}
body{
     background: #94cdde;
}
 *{
    box-sizing:border-box;
}
 .calculator{
     width: 100%;
     max-width: 280px;
     margin: 50px auto;
     border-radius: 10px;
     overflow: hidden;
     box-shadow: 0 0 5px #999999;
}
 .screen{
     background-color: orange;
     width: 100%;
     min-height: 100px;
     text-align: right;
     font-size: 24px;
     padding: 10px;
     padding-top: 20px;
}
 .cal-buttons{
     background: #ffffff;
     padding: 15px;
}
 ul#buttons{
     list-style-type: none;
     display: flex;
     flex-wrap: wrap;
     justify-content: space-between;
}
 ul#buttons li{
     font-size: 24px;
     color: #000000;
     background: #ecedef;
     width: 20%;
     margin:5px 2.5%;
     padding-top: 11px;
     height: 50px;
     border-radius: 50%;
     font-weight: bold;
     cursor: pointer;
     text-align: center;
     transition: all 0.3s;
}
 ul#buttons li:hover,ul#buttons li:visited{
     background: #777777;
     color: #ffffff;
}
 ul#buttons li:active{
     background: #000000;
}
 li#equals{
     width: 100% !important;
     border-radius: 10px !important;
}
</style>
</head>
<body>
<div class="calculator">
   <div class="screen">
      <h1 id="result"></h1>
   </div>
   <div class="cal-buttons">
      <ul id="buttons">
         <li>1</li>
         <li>2</li>
         <li>3</li>
         <li id="clear">C</li>
         <li>4</li>
         <li>5</li>
         <li>6</li>
         <li>+</li>
         <li>7</li>
         <li>8</li>
         <li>9</li>
         <li>-</li>
         <li>0</li>
         <li>x</li>
         <li>.</li>
         <li>÷</li>
         <li id="equals">=</li>
      </ul>
   </div>
</div>

<script>
var buttons = document.getElementById("buttons");
var li = buttons.querySelectorAll("li");
var result = document.getElementById("result");


for (let i = 0; i < li.length; i++) {
    li[i].addEventListener("click", function() {
        if (this.innerHTML == "=") {
            result.innerHTML = eval(result.innerHTML);
        } else {
            if (this.innerHTML == "÷") {
                result.innerHTML += "/";
            } else if (this.innerHTML == "x") {
                result.innerHTML += "*";
            } else if (this.innerHTML == 'C') {
                result.innerHTML = "";
            } else {
                result.innerHTML += this.innerHTML;
            }
        }
    });
}
</script>
</body>
</html>

​x
 
<!DOCTYPE html>
<html lang="en">
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Calculator App with JavaScript</title>
<style>
html, body, div, span, applet, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
a, abbr, acronym, address, big, cite, code,
del, dfn, em, img, ins, kbd, q, s, samp,
small, strike, strong, sub, sup, tt, var,
b, u, i, center,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, embed, 
figure, figcaption, footer, header, hgroup, 
menu, nav, output, ruby, section, summary,
time, mark, audio, video {
    margin: 0;
    padding: 0;
}
body{
     background: #94cdde;
}
 *{
    box-sizing:border-box;
}
 .calculator{
     width: 100%;
     max-width: 280px;
     margin: 50px auto;
     border-radius: 10px;
     overflow: hidden;
     box-shadow: 0 0 5px #999999;
}
 .screen{
     background-color: orange;
     width: 100%;
     min-height: 100px;
     text-align: right;
     font-size: 24px;
     padding: 10px;
     padding-top: 20px;
}
 .cal-buttons{
     background: #ffffff;
     padding: 15px;
}
 ul#buttons{
     list-style-type: none;
     display: flex;
     flex-wrap: wrap;
     justify-content: space-between;
}
 ul#buttons li{
     font-size: 24px;
     color: #000000;
     background: #ecedef;
     width: 20%;
     margin:5px 2.5%;
     padding-top: 11px;
     height: 50px;
     border-radius: 50%;
     font-weight: bold;
     cursor: pointer;
     text-align: center;
     transition: all 0.3s;
}
 ul#buttons li:hover,ul#buttons li:visited{
     background: #777777;
     color: #ffffff;
}
 ul#buttons li:active{
     background: #000000;
}
 li#equals{
     width: 100% !important;
     border-radius: 10px !important;
}
</style>
</head>
<body>
<div class="calculator">
   <div class="screen">
      <h1 id="result"></h1>
   </div>
   <div class="cal-buttons">
      <ul id="buttons">
         <li>1</li>
         <li>2</li>
         <li>3</li>
         <li id="clear">C</li>
         <li>4</li>
         <li>5</li>
         <li>6</li>
         <li>+</li>
         <li>7</li>
         <li>8</li>
         <li>9</li>
         <li>-</li>
         <li>0</li>
         <li>x</li>
         <li>.</li>
         <li>÷</li>
         <li id="equals">=</li>
      </ul>
   </div>
</div>
​
<script>
var buttons = document.getElementById("buttons");
var li = buttons.querySelectorAll("li");
var result = document.getElementById("result");
​
​
for (let i = 0; i < li.length; i++) {
    li[i].addEventListener("click", function() {
        if (this.innerHTML == "=") {
            result.innerHTML = eval(result.innerHTML);
        } else {
            if (this.innerHTML == "÷") {
                result.innerHTML += "/";
            } else if (this.innerHTML == "x") {
                result.innerHTML += "*";
            } else if (this.innerHTML == 'C') {
                result.innerHTML = "";
            } else {
                result.innerHTML += this.innerHTML;
            }
        }
    });
}
</script>
</body>
</html>